Doing something knotty (5)

I believe the answer is:

tying

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'doing' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for tying that I've seen before include "Binding, fastening" , "Restricting" , "restrictive" , "Scoring the same" , "Linking" .)

I've seen this clue in the Evening Standard.

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